Automatic gain control circuit



Oct. 17, 1961 C. H, TAYLOR AUTOMATIC GAIN CONTROL CIRCUIT Filed Sept. 30, 1958 2 Sheets-Sheet 1 INVENTOR.

./JHARLES H 721mm ffwm,

Oct. 17, 1961 c. H. TAYLOR AUTOMATIC CAIN CONTROL. CIRCUIT 2 Sheets-Sheet 2 Filed Sept. 30, 1958 United States Patent Office 3,005,094 Patented VOct. 17, 1 961 3,005,094 AUTOMATIC GAIN CONTROL CIRCUIT Charles H. Taylor, Haddoneld, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Sept. 30, 1958, Ser. No. 764,287 5 Claims. (Cl. Z50- 20) The present invention relates to automatic gain control (AGC) circuits, and more particularly Yto novel automatic gain control circuits particularly yuseful in connection with single sideband (SSB) receivers.

Known AGC systems are basically carrier controlled. In one type of known system a portion of the output of the input signal channel is rectified and used to `control gain in the input signal channel by adding rectified `voltages to the initial or xed bias of the amplifier stages in the channel. Another known system uses a separate amplier in parallel with the signal channel amplifier. The separate amplifier is usually of fixed gain and supplies an AGC rectifier with an input which is directly proportional to the channel amplifier input. 'Ihe voltage available at the AGC rectifier input is used to control gain in the signal input channel. In contrast to the foregoing, the novel AGC system of the present invention provides an AGC signal voltage which, in operation of electronic apparatus having an input signal channel and a following signal channel, is capable of holding the gain of the following signal channel substantially constant without subjecting the amplifier stages of the input signal channel to excessive cross modulation or overloading. This feature is, as indicated above, particularlyuseful in controlling gain in an SSB receiver.

Receivers for SSB suppressed carrier reception are usually designed to accept both upper and lower sidebands simultaneously. Useful but unrelated signals may then be received in both the upper and lower sideband portion of the acceptance spectrum simultaneously. The intermediate frequency (IF) amplifier must have a bandwidth at least twice that required by each sideband and the radio frequency (RF) amplifier bandwidth will generally exceed that of the I-F system. Use of known AGC systems in an SSB receiver is undesirable for several reasons. If the AGC control voltage is derived from the IF channel before the signal passes through the means for separating and selecting the upper and lower sidebands the gain of the system is controlled by a voltage proportional to the energy in the IF pass band. This energy may be due to several signals plus the desired one. When the desired signal input is small compared to the total signal in the channel, or to a large signal in the unwanted sideband, system gain can be so small as to result in a strength of desired signal which is not usa'ble. Deriving an AGC signal from the desired sideband can reduce the eect of attenuating a desired signal by a large signal in the undesired sideband. However, this is unsatisfactory because of overloading or blocking of the apparatus due to large signals in adjacent sidebands.

In accordance with the present invention an AGC voltage from each SSB channel is employed for comparison purposes. Each AGC signalV may be derived from any point in each of the SSB channels provided that the signal in the channel is within frequency range of an SSB signal. The frequency range in each SSB channel is determined by a band pass filter. Conveniently, each AGC signal is derived from a rectifying device and each of these rectifying devices provides two voltages opposite in polarity and proportional to the sideband signal to be controlled. These rectifying devices may be supplied with a bias to delay action until the signal in each sideband channel exceeds some predetermined value. In one embodiment of the invention, each sideband channel includes two amplifier sections. The iirst section is normally operating at or near its maximum gain. The gain in this rst section is reduced by the action of the rectifying device producing a negative AGC voltage when a large signal is present. The second amplifier section is supplied with a bias such that it operates at a low gain. A positive AGC voltage derived by comparison controls the second section of the amplifier by changing its bias to increase gain when necessary in order to hold the output constant. A ffeature of the present invention is the provision of an AGC voltage derived by comparison between or among AGC voltages developed in different channels. The derived AGC voltage is of appropriate polarity to exercise the desired gain control function.

If desired, an AGC rectifier may be employed to control the Vgain of RF and IF amplifiers which precede the means for separating and selecting the upper and lower sidebands. The purpose of controlling vthe gain of RF and IF 'amplifiers is to prevent overloading and cross modulation in early stages of receiver before the two sidebands are separated, and to hold the input to the sideband separating and selectingmeans at a reasonably constant level. The principal object of the present invention is to provide a novel AGC system for electronic apparatus having a wide signal acceptance band in an initial portion thereof followed by a plurality of signal channels each having a more restricted signal acceptance band.

Another object of the invention is to provide a novel AGC system for an SSB receiver whereby substantially to eliminate the effect of undesired received signals in operation of the receiver.

A further object of the present invention is to provide a novel AGC system for an SSB receiver in which an AGC signal derivedl from the total signal in the initial stage or stages of thereceiver is applied in a novel manner to aid in controlling the gain of the later narrower band stages of the'receiver, which are themselves provided with novel means to produce automatic gain control voltages for each of the later narrower band stages.

A still further object of the present invention is to provide, in a radio receiver having -a plurality of relatively narrow bandsignal channels, a novel gain control system operative to maintain the amplification capability equal and substantially constant in each of these relatively narrow channels.

Other objects and advantages of the present invention will, of cou'rse,`becomeapparent and immediately suggest themselves to those skilled in the art to which the inven- .tion is directed from a reading of the following specification in connection with the accompanying drawing in which:

l is a schematic diagram of an AGC system ernbodyin'g 'the present invention as incorporated in an SSB receiver; i

FIG. 2 is a .more detailed showing of the circuitry of the apparatus of'FIG. l;

fier 12. The amplifier 10 is supplied from a connection 6 with a signal received by an antenna or by :a radio or wire line signal relay system. ,In the usual SSB receiver, to which the invention is shown as being applied by way of example, the RF and IF stages accept upper. and lower sidebandsignals simultaneously. The IF lamplilier must have a bandwidth atleast twice. that required by thev` spectrum of each sideband and the RF amplifier bandwidth usually will exceed that of the IF amplifier. In ills discussion which is to follow it will be that tns receiver is designed for'. er is. capable of, receiving and suitably dcnioduating. suppressed carrier signals. However. as dernudulating airsngernents fersupprcsscd carrier signals are well'lrncwn. @sans for supplying .the dcnwduating Carricr locally isset. 1 i The total signal in. beth sideband regiensiias nell as incidental .uniss appcaring in .tctal'acgcptanceband ci the I'E ampliiier .12. appcarsat s voutput and is. shown. schematically. es being applied `'trs-finland 'pass .dltcrs VV.14 and 16- These dltcrscacli have an acceptance band. snitabi fur passing the upper and loner sidebandi respectively, O f a received signal centered alientan original carrier. The latter', however, supgujessd Kat; the originatiug transnn'ttcrius signal passed. by thc .itcr 14. naar bear no relationship to the signal passed by the i'dtr yig An AGC rectiiving-devicc17'oitnc usual trpc receives tnc total signal passed by the 1E amplifier 1,2.. `In accord: aus@ with Well known practice, an AGC voltagey dcivcd from, this rcctiying. device. is applied, as shown schinnatica llv. to the RF amplifier 1.0. and tothe. 'IF ampliiicr 12.. The AGCV voltage. derived Ifreni, the rcctifying; device-17 is, usually, negative with, respect to a. circuitircfcrencc pcin-t- The. more detailed slippingV of; indicates ground as the vreferencepaint v'Reference character. 18 designates a connection. later to bc described. to the AGC system of this invention.

The band pass tllter 14 has its out-p ut oonnectd to an amplier 19. which, las indicated by thlegend on the. drawiug, is .intends-d, without application of 1A.G..C. voltage, t operate at or near gain. u-t ofthe ampliiicr 19 is connected to 'an amplifier. '2..1wliicl1- is adjusted to operate at 1a. law gain level in the absence 0f an applied AGC. voltage Frein what! has.' been stated hcrcin; it will be. appreciated that a negative AGC vol-tage is tc be applied toi the. ampliiicr 19 and. a positive AGG voltage is` to be applied to. the amplifier liur cQnirQl, purposes operation ci. the system herein. Reference character .29 designates, schutV ally* an out.- put. ccnnccticn which. inay be. connected t. .igual utilizing. dev-icc such; fer e'icnn'ple,l as a .ctcctcrfanipliiienlnudf speaker combination. teleprintcr equipment., 'or the like. The output cf. the lter 1.6.. is connected te arnper 2.4. which is similar tc the. amplifier .149-5 Th. outriut of ysantuarios. Y

the amplifier 24 is connected to an amplifiorj'ld' Y is similar i9 amplicr 21... Ille. output. af.. tile'ansplisr 26 is indicated schematically by n connection 2.8. which may prcvidc signals teapparatus a similar tu.

that mentioned in describing tnccutput. cunnecucn @2.

fram the. ampliiicr 2.1-

VA rectifying device 31, to be described Vin moredotjail in ccuncritica. with. FlGf 2. rcccivcs.. over. a connection 22;, theV signal output of the amplilier 1 3 which, for errample,'represents the upper sideband signal. The rqctifying devicel provides fa positive outpnt voltage. yappeart ing in a Yconnection shown schematically at 32. i'This rectifying device. also provides. a negative voltage output appearing in a connection shown schematically lat 33.. In a similar manner,- a rectifying device 34 receives the signal output of the Iamplifier 24 over la connection indi- -cated schematically at 'lefhe rectifying"device 34 provides a positive voltage output to the connection indicated' schematically at 37, and a negative voltage output to the connection indicated schematically at 38'. The AGC voltage from the AGCy rectifying device 417', appearing in the previously mentioned connection 1 8, maybe applied to 'both rectifying devices 31V and 34 so .as to modify their combined operation in jointly controlling the gain in the channel following the band pass filter 14"a'n d the'han-nel following the band pass filter 16.

The positive AGC voltage 'appearing in the connection 37 is compared with the negative voltage appearing in the connection 33. This is accomplished in the illustrative example by means of resistors 41 and 42.. A connection 44 to the junction point 46 of these resistors applies the resultant AGC voltage tothe amplifier 21. It will be understood by those skilled in the yart that the schematic. arrangement illustrated by the resistors 41 and 42 may be in the nature of a potentiometer land that the connection 44 maybe made to the sliding connection of the potentiometer '(not shown) thereby to provide an additional adjustment Ifeature. f

The ccnnecticn 33 communicates with ya resister id A ,resistor 49 is connected at one end te the cunneeu'on 3 8. A connection 51 connects the junction point 5 2 of the resistors 48 and -49 to the amplifier 26,. As pointed out above. rcsistors. 48 and; 4? rnay also beiccmbincd into a single. potentiometer resistor.

ln operation of the arrangement shown in FIG. '1, it will rst be assumed that the incoming total signal applied Vto the RF amplifier 10 includes two SSB signals which substantially zero and there Ywill be no change in gain of the amplier 21 or the amplifier 26. More in detail, the rcctifying device 17-supplies -an AGC voltage to hold the. signal lat'. the point 15 Y'. subst'antially constant. The signal iat the pointvlS-includes everything the I F system is capable of passing. This is aboutr twice the bandwidth required for each sidebaud, as pointed out above. The amplifiers 1 9.,I 2lg 2.4 and 26 will be adjusted substantially to the, saine gain. 'Iheoutputs appearing in the connecf tions 29 and A28 will also be substantially equal.

When the upper and lower sideband signals accepted by the RP ampliier 10 are not equal, the. output of the rectifying devices will bel unequal. If the lower sideband 'sgIidL appearing in the output or" the. lter 16, is largo. compared to the upper sideband signal appearing in. the output of tho vfilter 14, the positive A GC voltage appearing in the connection 37 will exceed the negative vol-tage. in the connection 33 by .an amount proportional iQ 'the dilercuce between the signals. The resulting posi tivo voltage is u sed -to increase the gain of the amplitier 21 to. gives. normal Output- In describing FIG. 2 of theY drawing, similar reference choratcis. will be used, where applicable, to indicate similar 'or the same devices in. FIG. l without further detailed. description thereon `Anil-XGC. arnplier 55 is connec'tcd tc. the paint. 11S` and. is coupled by a transformer 5.6. to an A GC rectifier 58,V The elements 55, 56 andv 53 correspond in function .to the AGC rcctifying device 17 of FIG. 1. The anode of the rectifier 58 is connected to. a Potential, reference pointv such,` for example, asgrcund by Way Of. a, Vresistor 6.1.- Delaycd AGC voltage may be applied by way of a terminal 62J if desired, to produce a current ow in the resistor 61`thereby to develop an additional voltage drop. The circuit including the rectier 5 8 and the secondary of thev transformer 56 also includes a resistor 63 shoutedV by a capacitor 64 which develops the AGC voltage drop. A n AGC lter comprisiug'a resistor 66, and a capacitor 67 applies the suitably `filtered AGC voltage to the resistor 680i' a potentiometer. The connection 18 indicated in FIG. 2 corresponds to the connection 18 shown in FIG. l of the drawing..V One. end of the potentiometer resistor 68 is connected to ground which, in eiet, is the ground for the midpoint of resistors 71 and' 72. The resistorst71tand 72 are effectively shuntod by capacitorsV 81 and 82, respectively. y

The connection 22 vof FIG. 2'! corresponding to the conncction 22 of FIG. l', communicates with an AGC arn-A plifier 74. The output of the amplifier 74 is coupled to a transformer 76, the secondary of which is included in an AGC circuit comprising theresistor 71, a rectifier 78 and a resistor 79. AIhis AGC rectifying circuit is completed by way of ground connections. Resistor 79 is sh-unted by a capacitor 84. Various time constants may be used for the AGC circuits but in general they should be of the fmt operate-slow release type so as to give AGC voltages of this character in the combining circuits. A negative AGC voltage is taken from the anode of the rectifier 78 for connection to the resistor 41 as previously explained in connection with FIG. l of the drawing.

A circuit similar to the one previously described .for developing an AGC voltage includes the secondary of a transformer 86, the rectifier 88, a resistor 89 and the resistor 72. The resistor 89 is shunted by al capacitor 92. An AGC amplifier 94 receives its input from the connection 36 and its output supplies the primary of the transformer 86. The anode of the rectifier 88 communicates over the connection 38 with one end of the combining resistor 49.

The secondary of the transformer 76 also supplies an AGC rectifying circuit comprising a rectifier 91, a resistor 97 and the resistor 71. The resistor 97 is shunted by a capacitor 99. The positive AGC voltage developed at the cathode of the rectifier 91 is applied by way of the connection'32 to one end of the combining resistor 48. The junction 52 of the combining resistors 48 and 49 is connected to the amplifier 56 by.way of connection 51.

'In addition to the previously described AGC circuit comprising the Vrectier 88, the secondary of the transformer 86 supplies an AGC circuit comprising a rectifier 183, a resistor 104, and the resistor 72. The resistor 104 is shunted by a capacitor 106. The cathode of the rectifier 103 is connected through the connection 37 to one of the combiningvresistorsV 42.

The output connections 29 and 28 of the sideband channels are shown as connected to demodulators 108 and 109. The output of these demodulators, as indicated above, may be the original modulating signals at the inputs to the transmitter modulator or modulators and may be of any character affording the desired type of communication. From the discussion of FIG. l appearing above, it is believed that the operation of the more detailed showing of FIG. 2 will be fully understood.

FIG. 3 of the drawing shows a modification of the AGC system of FIGS. l and 2. In the description of FIG. 3 the reference characters appearingin FIGS. l and 3 will again be employed but with a suitable s-ufiix. The RF amplifier a receives signals from a suitable source such as an antenna and these signals are supplied to a mixer 11a. An oscillator 11b is associated in the usual manner with the mixer 11a. The latter supplies signals to an IF amplifier 12a and these signals in amplified form appear at the point a. The upper and lower sideband filters 14a and 16a, respectively, are fed with the signal appearing at the point 15a.

The upper sideband filter 14a feeds the selected upper sideband signal to an upper sideband amplifier 116. The lower sideband filter 16a feeds its signal to the lower sideband amplifier 118. The signal passed by the upper sideband filter 14a is also fed to an AGC amplifying and rectifying device 121. The signal appealing in the lower sideband filter 16a is fed to an AGC amplifying and rectifying device 122. The negative voltage loutput from the AGC amplifying and rectifying device 121 is compared with the positive voltage output of an AGC amplifying yand rectifying device 126 by means of resistors 128 and 129. The junction point 131 of these resistors is connected to provide an AGC voltage to the upper sideband amplifier 116. The character of this voltage is indicated on the abscissa of the plot in FIG. 4 of the drawing.

Voltage comparison is obtained in Va similar manner by resistors 133 and 134. 'Iheseresistors compare the positive AGC voltage output vof the AGC amplifying and rectifying device 126 with the negative output of the AGC amplifying and rectifying device 122. The junction point 136 of these resistors is Yconnected to the AGC amplifying and rectifying device 122. It will be understood that the AGC voltages discussed herein-are referred to a circuit reference point such, for example, as ground. The manner `in which this may be done is indicated more in detail in the illustrative example of FIG. 2 wherein AGC circuitry is shown more in detail.

Output signals from the rupper sideband amplifier 116 and the lower sideband amplifier 118 appear, respectively, in the connections 123 and 124. The latter correspond to the connections 22 and 28 of FIG. l ofthe drawing.

The total signal appearing at the point 15a, which is the Output signal of thewideband IF amplifier 12a, is applied to an AGC amplifying and rectifying device 126. The latter may correspond in circuit details to the arrangement showny in fFlG. 2 which is fed by the transformer 56. However, the amplifying and rectifying device 126 is of such a character that it provides a negative voltage -Ewl and a positive AGC voltage -l-Ew2. The RF and IF amplifiers are of conventional design operating at their maximum gain until some predetermined level is reached, after which gain is reduced by action of' the negative voltage -Ew1. The AGC system, in view of this, preferably opcrates as a delayed AGC system which is well known and need not be further described. The rectified outputs of the sideband AGC amplifiers are proportional to and in the same ratio as the corresponding signals at the input to the RF amplifier 10a. The sideband amplifiers 116 and 118 arev designed to operate at a mean gain sufficient to produce Vnormal output when the sideband signals and the D,.C. control voltages -Enl and Enz are equal. FIG. 4 of the drawing indicates the characteristic of these amplifiers and upon application of an AGC voltage obtained in the manner to be described. A semi-fixed cathode bias s ets the normal operating point of each amplifier as indicated on FIG. 4. v Operation of the embodiment of FIG. 3 will be described by explaining its action under three operating conditions. With equal sideband signals applied at the input of the RF amplifier 10a, the output of the filters 14a and 16a, the sideband AGC control voltages, and the sideband amplifier gains for each channel will be equal. The AGC control voltage is the difference between the positive rectified widebandvoltage Em and the negative voltage Em or Enz, dependingA on the sideband Each of these voltages will produce. the same value of difference voltage and in the same polarity when subtracted from Em. These difference voltages are the AGCcontrol voltages applied to the variable gainrsideband amplifiers' to control the out put of the channels. Y

Operation will now be' explained when there are unequalsignals in thevsideband portions of the total acceptance band of the RF `amplifier 1021. Aswas pointed out previously, the same signals will be impressed on the two lters 14a and 16a. The outputs of the filters will be in the same ratio las were the two signals at the input to the RFramplifierjla. l The action of the voltage -Ewl will be to hold the input tothe filters approximately constant over a wide range of vinput 'valuesr and this input will be about the same las is the case previously discussed when the sidebandv signals were equal.l The negative voltages Em and En, willbe of different values proportional to the outputs of the respective filters. The smaller one, representing 4the smaller signal will produce a positive difference voltage when subtracted from Ewz. This difference voltage willincrease with -a decrease in input signalto thefcorresponding sideband. The positive AGC control voltage 4will cause the sideband amplifier gain to be increased enough to produce standard output from that channel'.

Similar action will take place in the other channel. However, since the -si'gnal input is larger, the difference voltage will besrnaller and-.the gain/of thesideband au1- pler will be changed by a smaller amount. This result in the Output cf the two sideband amplifiers being apprexirnately equal. Y

When there is a signal in one sideband only of the acceptance Yband of the amplifier 10a, the signal input to the filters will be approximately off the same value ference voltage will be smaller and the sideband amplifier u will operate at its normal gain and produce a normal signal output. `The channel containing only noise will produce a small En voltage, resulting in a largeincrease in gain of the channel amplifier.V The gain of this channel will be at maximum when the RF and IF gain is kept at a low value due to a -large signal in the othersideband. Operation in the case just discussed is very similar to that discussed in the immediately foregoing when widely different signal levels for the two sidebands are present.

What is claimed is:

l. A circuitV for producing an' automatic gain control voltage for electronic apparatus having a single wide signal acceptance band means in Van initial portion thereof followed by a pair of signal channels each'having a more restricted signal acceptance band, means coupled to said wide signal acceptance band means for developing an automatic gain control voltage, means for separating said total signal into a pair of signals, means for feeding each of said pair of signals to a respective more restricted signal channel providing a pair of relatively narrow band signals, means for developing a' pair of voltages, each of said pair of voltages being proportional to said signal in the respective one of said restricted channels, means for applying said rst developed gain control voltage toY said means for developingv a pair of voltages, means for comparing Said pair of developed volta-ges vto develop a first comparison voltage becoming more positive as said signal strength increases in said first channel, means for again comparing said pair of developed voltages .to develop a second comparison voltage becoming more positive as Y Ysaid signal in said second channel increases, means for applying said first comparison voltage to said second chan.- nel to increase the gain thereof as said signal insaid first Y Y channel increases,l means for applying said second comparison voltage to said first. channel to increase the gain thereof as said signal in said second channel increases.

, 2.. ,In a radin receiver?Y having aLsinsle Wideband radin frequency amplifier, conversin means, a. single wideband intermediate frequency amplifier, an upper sideband channel lter and. a lower. channel filter; an automatic gain control system-comprising a Aleotfying device connected to said intermediate. frequency Y,amplifier, said rectifying device having means to providevanv automatic sain control voltage, van upper sideband am- @lifter a lower sideband amplifier, means associated with. each-respectivesideband filtrer to provide signals to a corresponding oneofrsaid each of said ampliiefs and. ccrrespendins et said. respective filters comprising Ycnc cf.Y a plural-ity ci sideband channels, a rectifying device connected-tc each ef said sideband channels tcreceive. signalsrthetctcms said. sideband. channel rectifying devices cach leaving means to. provide an antomatic gain control voltage, means kfor comparing said automatic gaincontrol voltage provided by said rectifying device connected to, said' intermediate frequency am- Y plifie'r with'said gain control voltage provided by said sideband channel rectify'ing devices. wherebyY to,v provide a plurality of comparison gain control voltages, and. means forY applying said comparison gaincontrol voltages toV control signal ygain in a respective' sideband channel.Y y

' 3. In a radio receiverrhaving a single-wideband radio frequency amplifica-conversion means, a single wideband intermediate frequency amplifier, and an upper sideband channel filter and a lower sideband channel filter; an automatic gain control system comprising a rectifying device connected to said intermediate frequency amplifier, said rectifying device having means to provide an automatic gain control voltage, means to apply said gain control voltage to said radio frequency amplifier and to said intermediate frequency amplifier, an upper sideband amplifier and a lower sideband amplifier, means associated with each respective sideband filter to provide Signals to a corresponding one of said amplifiers, each of said amplifiers and corresponding one of said respective filters comprising one of a plurality of sideband channels, a rectifying device connectedcto each of said sideband channels to receive signals therefrom, said sideband channel rectifying devices each having Vmeans to provide an automatic gain control voltage, means for comparing said automatic gain control voltage providedV by said rectifying device connected to said intermediate frequency amplifier with said gain control voltage provided by said sideband channel rectifying devices whereby to provide a plurality of comparison gain control voltages, and means for applying said comparison gain control voltages to control signalr gain in a respective sideband channel.

4. An automatic gain control system for a single sidebaud receiver, said receiver comprising a single wideband signal receiving and converting channel, an upper sideband filter, an upper sideband amplifier operatively coupled to said upper sideband filter to provide an upper sideband channel, a lower sideband filter, a lower sideband amplifier operatively coupledrto said lower sideband filter to provide a lower sideband channehau up- Y per sideband AGC amplifying Vand rectifying device,

means for supplying signals from said upper sideband channel to said upper sidebandV AGC amplifying and rectifying device, a lower sideband :AGC amplifying and rectifying device, means for supplying signals from said lower sideband channel to said lowery sideband AGC amplifying and rectifying device, a third AGC amplifying and rectifying device, means for supplying signals from said wideband signal receiving and converting channel to said third AGC amplifying and rectifying device, said third AGC .amplifying and rectifying device having means for developing a negative automatic gain control voltage, means for applying said negative automatic gain control voltage -to said wideband signal receiving and converting channel to control the gain thereof, said upper sideband AGC amplifying and rectfying device having means for developing a positive Vand. va negative AGC voltage, said lower sideband amplifying and rectifying device having means for developing a positive and a negative AGC voltage, means for comparingrsaid positive AGC voltage from said upper sideband AGC amplifying and rectifyingV device with said negativev AGC voltage from said lower sideband AGC amplifying and rectifying device to provide a Ifirst comparison AGC voltage, means operatively to apply said first comparison AGC voltage to said lower sideband channel, means for comparing said positive AGC voltage from said lower sideband AGC amplifyingV and rectifying device with said negative AGC voltage from said upper sideband amplifying device to provide a second comparison \AGC voltage, and means operatively to apply said second comparison AGC voltage to said upper sideband channel.

5. An automatic gain control systemV for a single sideband receiver, said receiver comprising a single wideband signal receiver, said receiver comprising a wideband band AGC amplifying and rectifying device, a lower sideband AGC amplifying and rectifying device, means for supplying signals from said lower sideband filter to Said lower AGC amplifying and rectifying device, a third AGC amplifying and rectifying device, means for supplying signals from said wideband signal receiving and converting channel to said third AGC amplifying and rectifying device, said third AGC amplifying and rectifying device having means for developing a negative automatic gain control voltage and means for developing a positive automatic gain control voltage, means for applying said negative automatic gain control Voltage to said wideband signal receiving and converting channel to control the gain thereof, means for comparing said last-named positive AGC Voltage with the AGC output of said upper sideband AGC amplifying and rectifying device thereby to develop a rst comparison AGC voltage, means for applying said rst comparison AGC volt- References Cited in the tile of this patent UNITED STATES PATENTS 2,303,542 Goddard Dec. 1, 1942.

2,400,073 Cawein May 14, 1946 2,491,918 -De Boer et al. Dec. 20, 1949 2,783,372 Peterson et al. Feb. 26, 1957 FOREIGN PATENTS 663,664 England Dec. 27, 1951 

